Detector of range of supply voltage in an integrated circuit

ABSTRACT

The disclosure relates to detectors of the level of supply voltage in an integrated circuit. The disclosed detector is designed to detect the crossing of low levels of supply voltage. It comprises a first arm to define a first reference voltage and a second arm to define a second reference voltage, these two reference voltages varying differently as a function of the supply voltage and their curves of variation intersecting for a value of the supply voltage located close to a desired threshold. A comparator receives the two reference voltages. The first arm has a resistive divider bridge, an intermediate connector of which constitutes the first reference voltage. The second arm comprises a resistor series-connected with a native P type MOS transistor, the point of junction of this resistor and this transistor constituting the second reference voltage. A non-linear element may be parallel-connected to the resistor which constitutes the first reference voltage.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to detectors of the level of supply voltageVcc of an integrated circuit.

[0003] These detectors are incorporated into the integrated circuit andcan be useful for various applications such as:

[0004] detecting that the supply voltage is in a specified range forwhich the circuit is designed and prohibiting operation if the voltageis in another range;

[0005] detecting the range in which the supply voltage is located ifthere are several possible ranges, and changing the configuration ofoperation of the integrated circuit as a function of the detected range;

[0006] ascertaining that the supply voltage has reached a specifiedthreshold before permitting the operation of the integrated circuit.

[0007] Thus, for example, it may be sought to make a voltage leveldetector that finds out whether or not the voltage level is greater thanabout 2 volts and another detector that finds out whether or not it isgreater than about 4 volts. These two detectors may be usedsimultaneously in one and the same integrated circuit which could workfor several ranges of supply voltages that are different, such as forexample a range of 2.7 to 3.3 volts and a range of 4.5 to 5.5 volts. Thecombination of information elements given by the two detectors indicatesthe range in which the supply voltage is located.

[0008] 2. Description of the Prior Art

[0009] Such a detector, shown in FIG. 1, consists of a comparator havingas input two reference voltages, Vref1 and Vref2, that vary differentlyas a function of the supply voltage, VCC, and that vary in such a waythat their curves of variation intersect when the supply voltage reachesa specified threshold (FIG. 2). The comparator compares these tworeferences and switches over in one direction or another depending onwhether the supply voltage Vcc crosses this threshold or not. The outputof the comparator may be applied through a buffer amplifier to the restof the integrated circuit in order to modify, permit or prohibit itsoperation depending on the desired application.

[0010] There are two main difficulties, which affect the detection oflow supply voltage levels (the detection of a threshold of 2.5 volts forexample). First, it is difficult to make a comparator that worksaccurately even when it is supplied with a low value of supply voltage(far below the threshold to be detected). Second, it is difficult tomake reference voltages that meet the above conditions (regardingdifferent variations as a function of Vcc, and threshold value ofintersection of their variation curves) because each reference voltagevaries as a function of Vcc which depends firstly on the operatingtemperature of the integrated circuit and secondly on the variations ofparameters of the method of manufacture of this

[0011] Consequently, whereas it is sought to have reference voltageswhose curves of variation intersect at a well-defined point thatcorresponds to a desired threshold value Vs, it is observed in realitythat it is necessary to plot a quadruple network of curves thatintersect in a zone of threshold values which may be very extensive.This quadruple network consists of two networks of curves for the firstreference voltage Vref1 and two networks for the second reference Vref2.For each reference, a network of curves may be plotted as a function ofthe possible variations of the manufacturing method and another may beplotted as a function of the operating temperature of the circuit.

[0012] It can be easily understood that, with this quadruple network,the variation of the threshold voltage as a function of manufacture andas a function of the temperature becomes great and makes the detector oflittle use and of little reliability.

[0013] This is all the truer as the curves of variation of the referencevoltages intersect with a narrower acute angle. For, the greater themanufacturing and temperature variations, the more variable will be theposition of the intersection.

[0014]FIG. 3 gives an exemplary illustration of the different points ofintersection of a curve Vref2 with several curves Vref1 corresponding toa certain degree of variation of manufacturing parameters and/or avariation of operating temperature. The result thereof is uncertainty inregard to the significance of the output information from the comparatorfor it corresponds to a crossing of a threshold Vs which may vary withina fairly broad range.

[0015] There is therefore need for a detector whose detection thresholdis as stable as possible despite variations in manufacture and despitevariations in operating temperature. This detector must be simple andmust consume little current.

SUMMARY OF THE INVENTION

[0016] The invention proposes a detector of this kind to detect thelevel of supply voltage Vcc of an integrated circuit, this detectorcomprising a first arm to define a first reference voltage Vref1 and asecond arm to define a second reference voltage Vref2 these tworeference voltages varying differently as a function of the supplyvoltage Vcc and their curves of variation intersecting for a value ofVcc located close to a desired threshold Vs, and a comparator receivingthe two reference voltages, wherein the first arm has a resistivedivider bridge, of which an intermediate connector constitutes the firstreference voltage Vref1 and the second arm comprises a resistorseries-connected with a native P type MOS transistor, the point ofjunction of this resistor and this transistor constituting the secondreference voltage.

[0017] It may be recalled that a native transistor, as opposed to adepleted or enhanced transistor, is a transistor formed in a dopedsemiconductor region, the channel of which has not undergone any surfacedepletion (P type doping for a PMOS transistor) or surface enhancement(N type doping for a PMOS transistor). The channel is therefore formeddirectly on the surface of the doped region without the performance ofan ion implantation or diffusion after the formation of the well. In thepresent case, the native transistor is a P type transistor and it isgenerally formed in an N type well diffused in a P type substrate.

[0018] The use of such a native transistor in the arm that defines thevoltage Vref2 in combination with the use of a resistive divider bridgein the arm that defines Vref1, leads to high stability of the thresholdvalue Vs of supply voltage Vcc that is to be detected. One of thereasons for this stability is the fact that the absence of doping of thechannel eliminates a factor of variation of characteristics in themanufacturing method. Furthermore, the use of a resistive dividerbridge, alone or complemented by a non-linear element, enables the veryefficient control, by a simple choice of relative values of resistance,of the zone of intersection of the curves of Vref1 and Vref2 as afunction of Vcc.

[0019] The native P type transistor is, in principle, mounted as adiode, namely with its gate connected to its drain. The gate and thedrain are then connected to the ground, while the resistorseries-connected with the transistor is connected between the source ofthe transistor and the supply Vcc.

[0020] In a particularly useful embodiment for making a detector with alow threshold level (about 2.5 volts), a non-linear element with lowconduction threshold voltage is connected in parallel to a resistor ofthe divider bridge (in practice the non-linear element is connected inparallel to the resistor at the terminals of which the voltage Vref1 istaken). The non-linear element is chosen so that the variation of Vref1as a function of the temperature compensates for the variation of Vref2as a function of the temperature, so that the curves Vref1 and Vref2intersect for a value of supply voltage substantially independent of thetemperature.

[0021] The non-linear element is preferably a series-connected assemblyof a PMOS transistor and an NMOS transistor (preferably native) mountedas a diode. The sum of their threshold voltages is preferably lower thanthe threshold voltage of the native P type transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Other characteristics and advantages of the invention shallappear from the following detailed description, made with reference tothe appended drawings, of which:

[0023]FIG. 1, already described, shows the general principle of avoltage supply level detector;

[0024]FIG. 2 shows the curves of variation of Vref1 and Vref2 as afunction of Vcc, intersecting for a threshold voltage Vs to be detected;

[0025]FIG. 3 shows the variation of the threshold voltage detected;

[0026]FIG. 4 shows a diagram of a detector according to the invention;

[0027]FIG. 5 shows a more detailed diagram;

[0028]FIG. 6 shows the curves of variation of Vref1 and Vref2 as afunction of time when a linearly increasing and then stable voltage Vccis applied;

[0029]FIG. 7 shows a variant of an embodiment for the detection of athreshold of about 2.5 volts;

[0030]FIG. 8 shows the corresponding curves of variation for Vref1 andVref2;

[0031]FIG. 9 shows a diagram of a comparator that can be used in thedetector according to the invention.

[0032]FIG. 10 shows a more complex embodiment of the invention, designedto detect higher voltage supply levels.

DETAILED DESCRIPTION

[0033] The general schematic drawing of the invention is shown in FIG.4. The detector has two arms supplied with the voltage Vcc, one used todefine a reference voltage Vref1 having a first type of variation as afunction of Vcc and the other used to define a reference voltage Vref2having a second type of variation as a function of Vcc. The two types ofvariation are different in order that the curves of variation mayintersect. The first arm has a divider bridge of resistors (R1, R2, R3)while the second arm has a resistor (R4) series-connected with a nativeP channel transistor T1. An intermediate connector of the divider bridgeof the first arm defines the reference Vref1. The point of junction ofthe resistor and the transistor of the second arm defines the referenceVref2. These two reference voltages are applied to the inputs of acomparator COMP which gives a logic signal at an output S. The signal atthe output S has a first level (for example a high level) when Vref2 isgreater than Vref1, which occurs if Vcc is below a threshold Vs. If not,it has a second level (low level).

[0034] In the embodiment described in FIG. 4, designed to detect anypassage of the supply voltage beyond a threshold of about 4 volts, thenative transistor P has its gate connected to the ground, its sourceconnected to the resistor R4 and its drain connected to the ground. Thesource is connected to the well in which, in a standard way, there isformed the transistor (integrated circuit on P type substrate, N typewell for the P channel transistors, and this well does not undergo anysurface implantation of depletion or enhancement).

[0035] The divider bridge here is formed by three series-connectedresistors, R1 connected to Vcc, R3 connected to the ground and R2connected between R1 and R3. The intermediate connector defining Vref1is the junction point of R2 and R3 The resistor R2 may beshort-circuited by a switch activated by the output of the comparator.It is used to set up hysteresis in changing the value of the divisionratio set up by the divider bridge depending on whether the comparatorswitches over in one direction or the other. Here, when Vcc is below thethreshold Vs, the resistor R2 is short-circuited, the division ratio isR3/(R1+R3). If Vcc goes beyond the threshold (which depends on thisdivision ratio), the comparator switches over and sets up the seriesconnection of the resistor R2, making the division ratio go toR3/(R1+R2+R3). This tends to lower the threshold Vs for which thecomparator switches over. Thus, the instabilities when the voltage Vccis close to the threshold are avoided.

[0036] The sum R1+R2+R3 is any sum that is low enough so as to avoidexcess consumption of current, for current consumption is a major factorto be watched in the designing of an integrated circuit.

[0037] Owing to the fact that Vref1 is set up, in this embodiment, onlyby the divider bridge to whose terminals the supply voltage Vcc isapplied, the reference voltage Vref1 varies linearly as a function ofVcc. The slope of this variation is R3/(R1+R3) or R3/(R1+R2+R3). Such aslope depends very little on the method of manufacture for it does notdepend on the value of the resistors (which are subject to highvariations of manufacture) but only on their ratio (which undergoes verysmall variations).

[0038] The ratio R3/(R1+R3) is chosen so as to set up a slope ofvariation of Vref1 as a function of Vcc which intersects the curve ofvariation of Vref2 for a value of Vcc equal to a desired threshold Vs,which in this case is in the range of 4 volts. The slope to be chosenfor the curve Vref1 as a function of Vcc to obtain this thresholddepends of course on the curve of variation of Vref2 and this curvedepends on the resistor R4 and the transistor T1. By a process ofempirical definition, it is possible to find values of R1, R3 and R4that are appropriate for obtaining a given threshold.

[0039] The transistor T1 has a threshold voltage Vtp0 of the order of1.6 volts. So long as Vcc remains below this value, the transistorremains off and the voltage Vref2 remains at the same value as Vcc. AsVcc increases, the transistor gradually becomes conductive and thecurrent in the resistor R4 increases essentially linearly with Vcc suchthat the voltage Vref2 increases substantially logarithmically with Vcc.The logarithmic curve obtained depends on the value of R4 and thegeometrical dimension (channel width/length ratio W/L) of thetransistor.

[0040] In practice, the detector shown schematically in FIG. 4 may haveother accessory elements. By way of an example, FIG. 5 shows a practicalembodiment in which there has been added a circuit designed to preventthe consumption of current when the integrated circuit is in a state ofstandby. An input terminal STBY transmits a standby signal which, whenit is at the high logic level, interrupts the consumption of current inthe two arms of the detector by means of the transistors T2 and T3 andinhibits the consumption of current and the working of the comparatorCOMP (by cancelling the signal for authorizing operation AMP_ON). Thetransistor T3 is inserted between the transistor T1 and the ground, andthe transistor T2 is inserted between the resistor R3 and the ground.Their influence on the working of the detector is negligible.

[0041] The switch which short-circuits the resistor R2 for thehysteresis may be formed in a standard way by an N type transistor T4parallel-connected with a P type transistor T5, one of these transistorsbeing controlled by the same logic level as that coming out of thecomparator and the other being controlled by the complementary logiclevel.

[0042] In an exemplary embodiment for which curves of variation of Vref1and Vref2 have been plotted in FIG. 6, the following values have beenchosen for the resistors and the transistor T1, with a view to obtaininga threshold voltage of about 4 volts: R1=52 kiloohms, R2=5 kiloohms,R3=200 kiloohms, R4=57 kiloohms and W/L=4.5/5.7 (micrometers) for T1.

[0043] The curves plotted in FIG. 6 show the variations of Vref1 andVref2 as a function of time while Vcc varies linearly as a function oftime and stabilizes at a value of 7 volts. This indirectly amounts to adepiction of Vref1 and Vref2 as a function of Vcc.

[0044] Since Vref1 in practice does not at all depend on the parametersof the method of manufacture or the operating temperature of theintegrated circuit, a single curve Vref1 has been plotted. However,since Vref2 depends on the variation of the parameters of the method ofmanufacture and also depends on the operating temperature, two extremecurves Vref2 a and Vref2 b have been plotted, representing the maximumvariation of the curve Vref2.

[0045] The intersection of the curve Vref1 and of the curve Vref2 acorresponds to a first value of Vcc substantially equal to 4.6 volts.The intersection of Vref1 and Vref2 b corresponds to a second value ofVcc, substantially equal to 3.3 volts.

[0046] Consequently, the comparator switches over and the detector givesa logic signal pertaining to a crossing of a threshold when Vcc exceeds4 volts, but with a margin of error of about 0.6 volts due to thevariations of manufacture and due to the temperature. This margin oferror is acceptable in an application where it is sought to find out ifthe supply voltage is rather in the 2.7-to-3.3 volt range or rather inthe 4.5-to-5.5 volt range.

[0047] With this same principle, but with a slightly modifiedembodiment, it is possible to detect a far lower threshold of supplyvoltage, substantially equal to 2 volts for example, with a smallermargin of error.

[0048] To detect a supply voltage of approximately 2 volts, the value ofthe resistor R4 and the size of the transistor T1 are increased toachieve an overall reduction of the voltage Vref2 which stabilizesswiftly in the neighborhood of the threshold voltage of the transistorT1 (about 1.6 volts). This results in a curve Vref2 a and a curve Vref2b which can be seen in FIG. 8 pertaining to a case where the resistor R4is equal to 500 kiloohms and the geometry of the transistor is W/L=20/2micrometers. The variation of the curve Vref2 comes above all from theoperating temperature of the integrated circuit.

[0049] The intersection of the curves Vref2 a and Vref2 b with a singlestraight line Vref1 as in FIG. 6 gives varied threshold values asexplained above, both for a detector of a threshold of about 4 volts andfor the detector of a threshold of about 2.5 volts.

[0050] However, this threshold variation can be reduced by adopting anadditional measurement, used in FIG. 7, consisting of the parallelconnection, to the resistor R3, of a non-linear element that plays norole so long as Vcc is low enough, but then attenuates the slope of thestraight line Vref1 in a non-linear way. This attenuation must depend onthe parameters of the method of manufacture as well as on the operatingtemperature of the integrated circuit, somewhat in the same way as thedependence of Vref2, so as to restrict the effective zone ofintersection of the curves Vref1 and Vref2.

[0051] In examining FIG. 8, it can be seen that a curve Vref1 a has beenplotted, representing a limit of variation of Vref1 as a function of themanufacturing parameters. This curve intersects the corresponding curveVref2 a (for the same extreme parameters of manufacture) at a pointcorresponding to Vcc=2.4 volts. And the curve Vref1 b intersects thecurve Vref2 b (for the other extreme of manufacturing parameters,) atanother point corresponding almost exactly to the same value 2.4 voltsfor Vcc.

[0052] Consequently, it will be understood that the range of error onthe threshold can be reduced to a very large extent, while at the sametime enabling an easy choice of the value of this threshold.

[0053] In the embodiment shown in FIG. 7, with reference to a 2.5 voltdetector, the non-linear element parallel-connected to the resistor R3is a series-connected assembly of two transistors mounted as a diode(with the gate connected to the drain), one being a P channel transistorT6 and the other being a native N channel transistor T7. Thesetransistors may be series-connected with a transistor T8 controlled likethe transistors T2 and T3 so as to be off in the standby mode. The sumof the threshold voltages of the transistors T6 and T7 is smaller thanthe threshold voltage of the transistor T1, so that the straight lineVref1 with a slope R3/(R3+R1) as a function of Vcc becomes curved (bythe gradual placing of T6 and T7 in a state of conduction) in the regionof intersection of the curves Vref1 and Vref2 namely for a value ofVref2 similar to the threshold voltage of T1 (1.6 volts in the examplegiven). For example, where the sum of the threshold voltages T6 and T7is about 1 volt to 1.3 volts (with a high dependence depending on thetemperature of the circuit), the dependence of Vref1 on the temperatureis fairly similar to the temperature dependence of Vref2.

[0054] Two additional transistors T9 and T10, activated by the standbysignal STBY, have been designed to place Vref2 initially at Vcc, inorder to ensure, unambiguously, that the operation of the circuit willstart in a condition where Vref2 is greater than Vref1.

[0055] One example of the numerical values that may be used to make alevel detector of about 2.5 volts, leading to the curves shown in FIG.8, is given here below:

[0056] R1=80 kiloohms, R2=20 kiloohms, R3=450 kiloohms,

[0057] R4=500 kiloohms,

[0058] geometry of T1: W/L=20/2

[0059] geometry of T6: W/L=15/2.4

[0060] geometry of T7: W/L=5/2.4.

[0061]FIG. 9 shows an example of a diagram of a comparator that can beused in the circuits of these above figures. This comparator can workeven for very low supply voltages of less than 2 volts. This comparatoris put into operation only outside standby periods, by means of a signalAMP_ON (the logic complement of the standby setting signal STBY). Thesignal AMP_ON permits or prohibits the passage of current in thedifferent arms of the comparator.

[0062] The integrated circuit may simultaneously include a detector suchas the one shown in FIG. 5 for the detection of a four-volt thresholdand a detector such as that of FIG. 7 for the detection of a 2.5-voltthreshold. The logic combination of the output indications from thesetwo detectors enables, for example, the very simple determination of therange of supply voltages in which Vcc is located, when it is known inadvance that the possible ranges are the following:

[0063] below 2.5 volts,

[0064] in the nominal range of 2.7 to 3.3 volts,

[0065] in the nominal range of 4.5 to 5.5 volts.

[0066] In a more complex embodiment, shown in FIG. 10, differentimprovements are provided. These improvements can be used separately ortogether, and seek above all to adapt the circuit to the detection ofhigher supply voltage levels.

[0067] In particular, in integrated circuits comprising electricallyprogrammable non-programmable memories, there is need for supplyvoltages in the range of 15 or 18 volts for the write circuits of thememory. These supply voltages are not given directly from the exteriorof the integrated circuit but are generally given by a so-called <<loadpump >> circuit, present in the integrated circuit itself and operatingthrough the one and only supply Vcc of the integrated circuit. It may benecessary to detect the level of the high supply voltage Hiv given bythe load pump.

[0068] The circuit of FIG. 10 enables this detection to be made. It hasdifferent parts, some of which receive the voltage Hiv to be detectedwhile others are supplied with the ordinary voltage Vcc (3 volts or 5volts for example).

[0069] The modifications of the circuit with respect to the schematicdrawing of FIG. 4 are as follows: first of all, the two arms used todefine the voltage references Vref1 and Vref2 are joined by a commonconductor, constituting a base for the two arms, this common conductorrepresenting a fictitious ground MF for which it will be seen that itspotential may be carried to the true ground of the circuit or to ahigher potential that may be chosen at will.

[0070] The resistors R1, R3 of the divider bridge of the first arm aremade in the form of two P channel transistors Q6, Q7 whose gates areconnected to the fictitious ground. The point of junction between thesetwo transistors is the one that defines the reference voltage Vref1. Aseries of diode-mounted transistors (with the drain connected to thesource), in this case a series constituted by P channel transistors Q2,Q3, Q4, Q5, is interposed in the first arm, between the fictitiousground and the resistive divider bridge Q6, Q7. These transistors Q2 toQ5 are used to set up a desired voltage drop between the fictitiousground and the resistive divider bridge. Their presence and their numberdepends on the application that is specifically envisaged.

[0071] Furthermore, it has been planned that there will be several Pchannel native transistors T1 a, T1 b, T1 c series-connected in thefirst arm and not just one native transistor T1.

[0072] In the second arm, the resistor R4 of the circuit of FIG. 4 isformed by a P channel transistor Q1 whose gate is connected to thefictitious ground.

[0073] To obtain a drop in potential between the level of the supplyvoltage Hiv and the junction point that defines the first referencevoltage Vref1 (the junction between the source of Q6 and the drain of Q7in the diagram of FIG. 10), there is provided, in the first arm, betweenthis point and the supply voltage Hiv, an element capable of causing adrop in potential, preferably a P channel transistor Q8 having its gateconnected to its drain and having high internal resistance (for example,with a length 50 times greater than its width). The drop in voltage maythus have a value of several volts. The transistor Q8 may be locatedbetween the transistor Q6 and the supply Hiv or between the transistorQ6 and the junction point that defines the reference Vref1.

[0074] A similar arrangement may be adopted in the second arm, with atransistor Q9 that brings about a drop of several volts in the voltagebetween the supply Hiv and the junction point that defines the secondreference Vref2. The transistor Q9 may be located between the transistorQ1 and the voltage Hiv or between the transistor Q1 and the referenceVref2.

[0075] With this type of arrangement, the reference voltages Vref1 andVref2 may be in the range of 7 to 8 volts. These voltage are however fartoo high to enable an operation of the comparator COMP. For, thiscomparator COMP is powered at a voltage Vcc of about 3 to 5 volts.

[0076] It is therefore preferably provided that the voltage Vref1 andthe voltage Vref2 will be converted, by stages of transposition ofvoltage level, into two other auxiliary reference voltages Vref′1 andVref′2, of a lower level (about 1.5 volts), and it is these auxiliaryreferences that are effectively applied to the inputs of the comparatorCOMP.

[0077] For this purpose, the voltage Vref1 is applied to the gate of ahighly resistive N channel transistor Q10, whose source is at theelectrical ground (and not at the fictitious ground MF) and whose drainis connected by a resistor R5 to the low supply voltage Vcc. Theauxiliary reference voltage Vref′2 is taken at the junction of thetransistor Q10 and the resistor R5.

[0078] Similarly, the voltage Vref2 is applied to the gate of a highlyresistive N channel transistor Q11, whose source is at the electricalground and whose drain is connected by a resistor R6 to the low supplyvoltage Vcc. The auxiliary reference voltage Vref′2 is taken at thejunction of the transistor Q10 and the resistor R5.

[0079] Finally, to enable, if desired, an adaptation of the detector tothe various ranges of possible values of voltage levels to be detected,it is provided that the potential of the fictitious ground MF may beoffset with respect to the ground of the integrated circuit, through aset of arms that can selectively connect the fictitious ground to theground under the control of a selection circuit. The different arms setup different voltage drops when they are put into operation. A selectedarm will be put into operation by the operation of turning on atransistor located at the foot of this arm (transistors Q20 to Q25 inthe diagram of FIG. 10 which has six possible arms). A selection signalSEL activates the operation of turning on a selected arm (in principleonly one arm at a time).

[0080] The different voltage drops are set up by different transistorsmounted in these arms (it is assumed that the base transistor introducesno voltage drop or a very low and identical voltage drop in all thearms). In the example shown, designed for the precise adjustment of thevoltage level detected, there are provided six arms in which the voltagedrops introduced are due to one or more series-connected N channeltransistors that are diode-mounted. These arms are the following:

[0081] arm A: direct link to place the fictitious ground MF at the realground;

[0082] arm B: drop in voltage of a native transistor Q26;

[0083] arm C: drop in voltage of an enhanced transistor Q27;

[0084] arm D: drop in voltage of an enhanced transistor Q28series-connected with a native transistor Q29;

[0085] arm E: drop in voltage of two enhanced transistors Q30 and Q31;

[0086] arm F: drop in voltage of two enhanced transistors Q32 and Q33and one native transistor Q34 or two native transistors and one enhancedtransistor;

[0087] and so on and so forth, other arms could be planned, with theparallel connection of several arms itself possibly providing forgreater precision of adjustment of the levels to be detected.

[0088] Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. A detector of the level of supply voltage Vcc ofan integrated circuit, comprising a first arm to define a firstreference voltage and a second arm to define a second reference voltage,these two reference voltages varying differently as a function of asupply voltage and their curves of variation intersecting for a value ofthe supply voltage close to a desired threshold, and a comparatorreceiving the two reference voltages, wherein the first arm has aresistive divider bridge, of which an intermediate connector constitutesthe first reference voltage and the second arm comprises a resistorseries-connected with a native P type MOS transistor, the point ofjunction of this resistor and this transistor constituting the secondreference voltage, the native transistor in the arm that defines thesecond reference voltage, in combination with the resistive dividerbridge in the arm that defines the first reference voltage, provideshigh stability of the threshold value of supply voltage Vcc that is tobe detected, the resistive divider bridge, providing very efficientcontrol, by a simple choice of relative values of resistance, of thezone of intersection of the curves of the first and second referencevoltages as a function of a supply voltage Vcc, and wherein the two armsare joined by a connector defining a common potential for the base ofthe two arms.
 2. A detector according to claim 1, wherein the first armcomprises, between the base and the resistive divider bridge, one ormore series-connected transistors, with their drain connected to theirgate.
 3. A detector according to claim 1, wherein at least oneadditional transistor having its gate connected to its drain isseries-connected in each of the arms, between a supply whose level is tobe detected and the point that defines each of the reference voltages.4. A detector according to claim 1 comprising, between the commonconductor forming the base of the arms and a ground, several assembliesof transistors making it possible to set up, as desired, differentvoltage drops between the ground and the common base of the two arms, soas to enable an adjustment of the voltage levels that the detector maydetect, the placing of the different assemblies in a state of conductionbeing activated by individual selection, according to the desiredvoltage drop.
 5. A detector according to claim 1 for application to thedetection of high voltage levels wherein, between each of the pointsdefining the reference voltage levels wherein, between each of thepoints defining the reference voltages and the corresponding input ofthe comparator, there is provided a voltage level transposition stagecomprising a transistor and a resistor series-connected with thistransistor between a main low-voltage supply source and a ground, thefirst and second reference voltages being connected to the gate of thistransistor, the respective input of the comparator being connected tothe drain and to the resistor, the source of the transistor beingconnected to the ground.
 6. A detector of the level of supply voltage ofan integrated circuit, comprising: a comparator having a first inputreceiving a first reference voltage and a second input receiving asecond reference voltage, the two reference voltages varying differentlyas a function of a supply voltage to form curves of variation whichintersect at a value of supply voltage which is located close to adesired threshold, the comparator further having an output whichswitches from a first state to a second state when the supply voltagecrosses the threshold; a resistive divider bridge supplied with thesupply voltage, having an intermediate connector of which constitutesthe first reference voltage; and a resistor series-connected with anative P type MOS transistor, supplied with the supply voltage, a pointof junction of the resistor and the transistor constituting the secondreference voltage.
 7. A detector according to claim 6, wherein thenative P type transistor has a gate, source and drain, where the gate isconnected to the drain.
 8. A detector according to claim 7, wherein thegate and the drain are connected to the ground and the resistor which isseries-connected with the transistor is connected between the source ofthe transistor and the supply voltage.
 9. A detector according to claim6, wherein a non-linear element with relatively low conduction thresholdvoltage is parallel-connected to a resistor of the divider bridge.
 10. Adetector according to claim 9, wherein the non-linear element isparallel-connected to the resistor of the divider bridge at theterminals of which the first reference voltage is taken.
 11. A detectoraccording to claim 9, wherein the non-linear element has conductionthreshold characteristics that vary as a function of the temperature insuch a way that the curve of variation of the first reference voltagehas substantially similar temperature dependency as the curve ofvariation of the second reference voltage such that the first and secondcurves intersect at a value which is substantially independent of thetemperature.
 12. A detector according to 9, wherein the p type nativetransistor has a first conduction threshold voltage and wherein thenon-linear element has a second conduction threshold voltage lower thanthe first conduction threshold voltage.
 13. A detector according to oneof the claim 9, wherein the non-linear element comprises a seriesassembly of an N channel transistor and a P channel transistor.
 14. Adetector according to claim 13, wherein the N channel transistor is anative type of transistor.
 15. A detector according to claim 6, whereinthe resistive divider bridge comprises a bridge resistor having means toshort circuit the bridge resistor as a function of the output of thecomparator, in order to introduce hysteresis in the desired threshold.16. A detector of the level of supply voltage of an integrated circuit,comprising: a device for comparing having a first input receiving afirst reference voltage and a second input receiving a second referencevoltage, the two reference voltages varying differently as a function ofa supply voltage to form curves of variation which intersect at a valueof supply voltage which is located close to a desired threshold, thedevice for comparing further having an output which switches from afirst state to a second state when the supply voltage crosses thethreshold; a resistive divider bridge, supplied with the supply voltage,having an intermediate connector which supplies the first referencevoltage; and a resistor series-connected with a first nonlinear element,the series-connection being supplied with the supply voltage, the pointof junction of the resistor and the first non-linear element suppliesthe second reference voltage.
 17. A detector as in claim 16, wherein thenon-linear element is a MOS transistor which has no channel doping. 18.A detector as in claim 16, wherein the resistive divider bridgecomprises three series connected bridge resistors, the first bridgeresistor is connected to the supply voltage, the third bridge resistoris connected to a ground and the second bridge resistor is connectedbetween the first and third bridge resistors, the intermediate connectorwhich provides the first reference voltage being connected between thesecond and third resistors.
 19. A detector as in claim 18, wherein thesecond bridge resistor has short circuit means activated by the outputof the comparator in order to lower the threshold voltage for which thecomparator switches from the first state to the second state.
 20. Adetector as in claim 16, wherein the first nonlinear element has a firstconduction threshold voltage, and wherein a second nonlinear element,connected across the third bridge resistor, has a second conductionthreshold voltage smaller than the first conduction threshold voltage.21. A detector as in claim 20, wherein the second nonlinear elementvaries the first reference voltage as a function of temperature tocompensate for the variations of the second reference voltage as afunction of temperature, such that the curves of variation intersect ata value of supply voltage substantially independent of the temperature.22. A detector as in claim 16, wherein respective values of the bridgeresistors and the resistor which is series-connected with the firstnonlinear element determine the value of the supply voltage for whichthe curves of variation intersect.
 23. A detector as in claim 16,wherein an increased value of the resistor which is series-connectedwith the first nonlinear element and/or changes in size of the nonlinearelement provide a detector which detects relatively lower voltages.